1. Field of the Invention
This invention relates to a control system for internal combustion engines equipped with exhaust gas recirculation systems, and more particularly to a control system of this kind which controls engine control parameters by taking into consideration the influence of exhaust gas recirculation.
2. Prior Art
In internal combustion engines equipped with exhaust gas recirculation systems, it has been conventionally known to correct the supply fuel amount during operation of the exhaust gas recirculation system. Usually, an exhaust gas recirculation system has a recirculation control valve arranged across an exhaust gas recirculation passage. The control valve has a response time lag in being actuated by a control signal. In view of this response time lag, a control system of this kind has been proposed, which delays the timing of starting or terminating the correction of the supply fuel amount by a time period depending upon operating conditions of the engine, from the time the recirculation value is changed over from a closed position thereof to an open position or vice versa (Japanese Provisional Patent Publication (Kokai) No. 1-203641).
On the other hand, in conventional internal combustion engines of the type that fuel is injected into an intake pipe, there is a problem that some of injected fuel adheres to the inner surface of the intake pipe, so that a required amount of fuel cannot be drawn into the combustion chamber. To solve this problem, there has been proposed a fuel supply control method which estimates a fuel amount which is to adhere to the inner surface of the intake pipe and one which is to be drawn into the combustion chamber by evaporation from the fuel adhering to the intake pipe, and determines a fuel injection amount in dependence on the estimated fuel amounts (U.S. Pat. No. 4,939,658).
However, none of conventional internal combustion engines contemplate both the influence of the fuel adhering to the inner surface of the intake pipe and the influence of the exhaust gas recirculation upon the accuracy of control of the air-fuel ratio of a mixture supplied to the combustion chambers of the engines. This results in difficulties in accurately controlling the air-fuel ratio to a desired value.
Further, the above-mentioned proposed control system does not contemplate the time lag between the time the recirculation control valve is actually operated and the time the exhaust gas passing the valve actually reaches the combustion chamber, nor the influence of the recirculating gas temporarily staying in a bulk chamber provided in the recirculation passage. Therefore, there remains room for improvement in accurately controlling the air-fuel ratio of the mixture supplied to the combustion chamber and the ignition timing of the engine.
Particularly, the proposed control system does not contemplate the above-mentioned influence of the exhaust gas recirculation caused during a transient state of the engine such as at the start or termination of the exhaust gas recirculation, which results in degraded exhaust emission characteristics and driveability of the engine in such a transient state.